Electronic color cameras are known in which a single-chip image sensor is provided with an array of color filter elements to render the image sensor color-responsive. Such electronic color cameras employing single-chip solid-state color image sensors have the problem of producing colored fringes around high frequency image detail. This problem results because the different colors of the image are sampled at different parts of the image. When an image is reconstructed from the sampled image signals produced by the image sensor, generally some form of interpolation is employed for supplying interpolated values between the sampled color values. The interpolation of each color of the image is slightly different due to the different sampling locations, thus resulting in the colored fringes.
To illustrate the phenomenon, Table I is provided to represent the relative intensities of red, green and blue colors in one horizontal line across a vertical neutral colored edge.
TABLE I ______________________________________ Original Scene - Relative Intensities ______________________________________ Red 10 10 10 10 2 2 2 2 2 Green 10 10 10 10 2 2 2 2 2 Blue 10 10 10 10 2 2 2 2 2 ______________________________________
Table II shows the relative intensities sampled by an image sensor having a repeating pattern of red, green, blue, green sensing elements.
TABLE II ______________________________________ Sampled Values ______________________________________ Red 10 -- -- -- 2 -- -- -- 2 Green -- 10 -- 10 -- 2 -- 2 -- Blue -- -- 10 -- -- -- 2 -- -- ______________________________________
Table III shows the relative intensity values produced by linear interpolation between the sampled values in Table II.
TABLE III ______________________________________ Linear Interpolation Between Sampled Values ______________________________________ Red 10 8 6 4 2 2 2 2 2 Green 10 10 10 10 6 2 2 2 2 Blue 10 10 10 8 6 4 2 2 2 ______________________________________
As can been seen in Table III, the reproduced neutral edge is rendered in a variety of colors, ranging from a cyan color (green+blue) on the left to a blue color on the right.
One technique for reducing the appearance of these colored fringes employs signal processing on the sampled colored signals. For example, U.S. Pat. No. 4,176,373 issued to P. L. P. Dillon et al. on Nov. 27, 1979, shows a signal processing technique where a high frequency portion of a luminance signal is added to the color signals in an attempt to drive the image signal towards neutral at image details, thereby reducing the appearance of colored fringes.
Another technique for reducing the appearance of colored fringes employs an optical spatial frequency filter made from sheets of birefringent material. The optical spatial frequency filter splits an image into two or more slightly displaced images. The filter has the effect of blurring the input image. This technique is described in a published European Patent Application Publication No. 0-094-676 filed May 17, 1983, by Hitachi, Limited. Although both techniques are effective for reducing the appearance of colored fringes, neither technique alone or in combination completely eliminates the appearance of color fringes in the images reproduced from the sampled color image signals.